Precision laser machining apparatus

ABSTRACT

A focused laser beam having an optical axis passes through a first lens mounted to a first galvanometer and a second lens mounted to a second galvanometer. The first galvanometer is adapted to tilt the first lens about a X axis and the second galvanometer is adapted to tilt the second lens about a Y axis. This displaces the focused laser beam in a controlled manner from the optical axis to enable laser machining of very precise geometric features over a large processing window. In a preferred embodiment, the first and second lenses are a pair of inverted positive meniscus lenses, of high index of refraction material.

CROSS-REFERENCE TO RELATED DISCLOSURES

This disclosure is a divisional application claiming the benefit of thefiling date of March 2009 U.S. patent application entitled: “PrecisionLaser Machining Apparatus,” by the same inventors, filed Feb. 14, 2007,bearing Ser. No. 11/674,730, now U.S. Pat. No. 7,489,429.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical scanning devices used for lasermachining. More particularly, it relates to a high precision laser holedrilling and controlled material removal of geometries of less than 500microns.

2. Description of the Prior Art

Galvanometer scanners have been used for nearly three (3) decades forlaser material processing. They are most commonly used for lasermarking. They have less utility in fine machining applications, inparticular drilling precision holes and features below 250 microns,because their positional accuracy is limited below such threshold.Non-galvo-based approaches are limited to circular features and tend tobe more optically and mechanically complex. Galvanometer-based systemsare the simplest and least expensive way to direct a focused laser beamover a wide area. Nevertheless, they lack the “localized” precision forfinite features over a large field.

Thus there is a need for a galvanometer-based apparatus that includeslocalized precision for finite features over a large field.

The prior art also includes an air bearing X-Y stage that moves under afixed focused laser beam, providing precision and accuracy. However, airbearing devices are expensive and subject to high inertia arising frommoving such stages and the part supported by the stage.

Thus there is a need for a structure that provides the precision andaccuracy of an air bearing X-Y stage without the high cost and theinherent inertia. A conventional multi-mirror galvometric systempositions a focused laser beam by moving the mirrors by means ofvectors. There are no “true arcs” generated for circular features.Instead, a circle is approximated by a series of short vectors. It isvery difficult to form precision holes or any arc feature below 200microns in diameter. Moreover, the angular resolution of the galvomotors is a further hindrance to the problem of small features and theattainment of high repeatability. Thus there is a need for a system thatis not detrimentally affected by the formation of circles and arcsthrough a series of short vectors.

The known systems are also subject to limited angular resolution andthermal drift which further hinders the ability of the device to machineprecision features over a long period of time, e.g., a single productionshift in manufacturing.

There is a need, therefore, for a system that is less subject to theeffects of limited angular resolution and thermal drift so that featurescan be machined over relatively long periods of time.

A need therefore exists for a device having an improved angularresolution relative to the known devices that have a pair ofgalvometrically driven mirrors.

Rotating, offset wedge pairs allow good precision below 250 microns, butthey only permit circular features and have a limited dynamic range. Thefocus lens itself can be placed offset from the optical axis and rotatedor even placed in an open frame X-Y stage used to make all conceivablegeometries but mounting a lens in such a way is bulky and limited overthe area that can be machined due to common lens aberrations.

There is thus a need for a device not subject to the limitations ofrotating, offset wedge pairs or a rotating, offset focus lens.

Another known method, disclosed in U.S. Pat. No. 4,079,230, includes apair of matched optical wedges that are rotationally offset and rotatedin unison at high speeds. The offset of the matched wedges causes anangular displacement of the laser beam from the original optical axis.This angular deviation causes a lateral displacement of the focal spotwhen the angularly displaced beam is passed through a focus lens. Thedifficulty with this technique is that it is hard to coordinate the twowedges precisely at the high rotational speeds or to rapidly change thedesired angle of deviation during such rotation. This technique usuallyrequires a multitude of wedge pairs to cover a wide diameter range. Therequirement to change wedge pairs adds significant time to replace andalign; it is therefore unsatisfactory for most production processes.This method also limits the geometries to circular patterns only.

Other methods include “wobble plates” disclosed in U.S. Pat. Nos.4,940,881 and 6,501,045 that provide circular and tapered features onlyand have limited workability in imaged based optical systems.

Another method, disclosed in U.S. Pat. No. 4,896,944, employs an offsetfocus lens that is rotated to displace the focused spot radially fromthe optical axis. Such systems are bulky but have utility when fixeddiameter holes are required. They lack utility in creating complexfeatures or tapers.

Thus there is a need for a system that has increased versatilityrelative to the known systems. More specifically, there is a need for asystem that can provide complex shapes, including tapers and othernon-circular shapes, and which can form features less than two hundredfifty microns (250 μm).

However, in view of the art considered as a whole at the time thepresent invention was made, it was not obvious to those of ordinaryskill in this art how the identified needs could be met.

SUMMARY OF INVENTION

This invention provides a simple optical, electromechanical and softwareapproach to directing a focused laser beam onto materials to machinesimple and complex geometries. The novel structure provides the ease ofuse and simplicity of a galvo system but adds the “localized” precisionlacking heretofore.

The novel structure provides the precision and accuracy comparable to anair bearing X-Y stage that moves under a fixed focused laser beam butwithout the high cost and higher inertia of moving such stage and thepart. The present invention, in essence, demagnifies the scan field bymore than two (2) orders of magnitude.

A common scanner with fair resolution can have a scan field of 50 mm×50mm with an F-Theta lens having a focal length of 100 mm. This samescanner has great difficulty providing high accuracy of geometries below250 micron, due to the angular resolution of the system and the factthat any curved features include a large number of straight vectors.

The apparatus of this invention uses the same control of thegalvanometer but adds an optical demagnification that essentially mapsthe 50 mm×50 mm field into a 0.2 mm×0.2 mm field. This is accomplishedby a laser galvo scanner that reflects a laser beam over an angularrange of plus or minus (+/−) twelve to twenty degrees (12-20°) as thebeam passes through a focusing lens, typically an F-theta lens. Theangular repeatability of such a galvo is typically <+/−22 urad, whichrepresents a resolution of ˜+/−2.2 um for a scan lens having a 100 mmfocal length. The field of such a system will be f*(Tan Θ), where f isthe focal length of the lens and theta is the angle the beam isreflected before the lens. A laser scanner operating then over a plus orminus twelve degree (+/−12°) range with a f=100 mm lens will focus overa range of +/−21.3 mm. The angular deviation of a beam refracted througha thin optic is determined by the index of refraction of the materialand the angle the optic is tilted. Tilting a two millimeter (2 mm) thickoptical plate that has an index of refraction of 1.796 over a range ofplus or minus twelve degrees (+/−12°) degrees will cause a laser beamtraveling on axis through said plate to deviate from the optical axis byplus or minus 0.188 mm. This reduces the resolution of the same galvo bythe ratio of 21.2 mm/0.188 mm=113. This provides the benefit of thelarge field of a typical galvo scanner but adds another level ofaccuracy to the finite features within the large field.

The novel apparatus also compensates for irregularities in the focusedlaser beam. If a focal spot is elliptical, for example, the scanner isprogrammed to move in an opposing elliptical manner to compensate andachieve a perfectly round hole.

The primary object of this invention is to create an optical system thatprecisely and repeatedly locates a concentrated laser beam.

A closely related object is to manipulate the beam laser in such a wayas to remove a wide variety of materials in a controlled way to generatecomplex geometries with excellent precision and repeatability.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is a diagram of a laser beam passing through a pair of tilted,inverted meniscus lenses in accordance with the first embodiment;

FIG. 2A is a diagram depicting the tilted, inverted meniscus lens pairof FIG. 1 in an X-Z plane where the Z plane represents the optical axis;

FIG. 2B is a diagram depicting the tilted, inverted meniscus lens pairof FIG. 1 in a Y-Z plane where the Z plane represents the optical axis;

FIG. 3A is a magnified ray trace of the X-Z plane as light passesthrough the tilted, inverted meniscus lens pair;

FIG. 3B depicts how the light is deviated ˜120 microns from the opticalaxis from the corresponding tilt of lens 12 in FIG. 3A;

FIG. 3C is a magnified ray trace of the Y-Z planes as light passesthrough the tilted, inverted meniscus lens pair;

FIG. 3D depicts how the light is deviated from the optical axis from thecorresponding tilt of lens 14 in FIG. 3C;

FIG. 4A diagrammatically depicts the second embodiment for preciselydisplacing a focused laser beam from the optical axis over a very largefield, illustrating the X-Z axis plane view;

FIG. 4B diagrammatically depicts the second embodiment for preciselydisplacing a focused laser beam from the optical axis over a very largefield, illustrating the Y-Z axis plane view;

FIG. 5 diagrammatically depicts a third embodiment;

FIG. 6 depicts an array of circular holes machined in stainless steelwith the preferred embodiment;

FIG. 7 depicts an array of circular holes machined in stainless steelwith the ScanLab Hurryscan II of the prior art;

FIG. 8A depicts a circular geometrical figure machined in stainlesssteel with the second preferred embodiment;

FIG. 8B depicts a square geometrical figure machined in stainless steelwith the second preferred embodiment;

FIG. 8C depicts a triangular geometrical figure machined in stainlesssteel with the second preferred embodiment;

FIG. 8D depicts an irregular geometrical figure formed by machining instainless steel a second triangle atop a first triangle with the secondpreferred embodiment;

FIG. 9A depicts a circular geometrical figure machined in stainlesssteel with a calibrated ScanLab Hurryscan II scanner of the prior art;

FIG. 9B depicts a square geometrical figure machined in stainless steelwith a calibrated ScanLab Hurryscan II scanner of the prior art;

FIG. 9C depicts a triangular geometrical figure machined in stainlesssteel with a calibrated ScanLab Hurryscan II scanner of the prior art;and

FIG. 9D depicts an irregular geometrical figure formed by machining instainless steel a second triangle atop a first triangle with acalibrated ScanLab Hurryscan II scanner of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there it will be seen that the first embodimentof the invention is denoted as a whole by the reference numeral 10.

Incoming laser beam 11 passes through a pair of inverted positivemeniscus lenses 12 and 14. Each lens is mounted to a galvanometer totilt each lens perpendicular to one another to displace the focus laserspot from the original optical axis. The preferred optical material isthe highest possible index material for the desired laser wavelength.Having a high index allows the thickness of the lenses to be as thin aspossible to minimize optical aberrations and minimize the inertia on thegalvo.

In FIG. 2A lens 12 is tilted about the Y axis to displace the focusedlaser beam in a controlled manner along the X axis from the originaloptical axis.

In FIG. 2B, lens 14 is tilted about the X axis to displace the focusedlaser beam in a controlled manner along the Y axis from the originaloptical axis.

An amplified ray trace of the X-Z plane as light passes through thetilted, inverted meniscus lens pair is depicted in FIG. 3A.

FIG. 3B depicts by enlarging the ray trace at the focal point of thelens system how the light is deviated from the optical axis from thecorresponding tilt of lens 12 in the X plane in FIG. 3A.

An amplified ray trace of the Y-Z plane as light passes through thetilted, inverted meniscus lens pair is depicted in FIG. 3C.

FIG. 3D depicts by enlarging the ray trace at the focal point of thelens system how the light is deviated from the optical axis from thecorresponding tilt of lens 14 in the Y plane in FIG. 7C.

FIGS. 4A and 4B depict a second embodiment for displacing a focusedlaser beam from an optical axis. A laser beam enters a galvo scannerdevice that includes first and second galvo mirrors 16 a, 16 b. The beamis reflected from first galvo mirror 16 a onto second galvo mirror 16 b.From second galvo mirror 16 b, the beam passes through F-theta lens 18which typically includes three (3) lens elements as depicted in thedrawing surrounded by a dashed rectangle. The beam exits F-theta lens 18and passes through a pair of parallel plates 20, 22, each of which ismounted to a galvanometer. The parallel plate galvanometers areorientated orthogonally to one another so that the beam can be offsetfrom the optical axis in a controlled way. The offset of the beam isdetermined by the angle of the plate, its thickness and the index ofrefraction of the plate. The dashed lines at the focal point of the lensrepresent the original optical axis. FIG. 8A depicts the X-Z plane andFIG. 8B depicts the Y-Z plane. Parallel plates 20, 22 are tilted inFIGS. 8A and 8B by ten degrees (10°) for illustration purposes. Thecombination of the laser scanner and the galvanometer driven parallelplates allows precise features to be machined over a very large field.

FIG. 5 depicts a third embodiment that is an optically equivalentconfiguration to the preferred embodiment. A laser beam passes through asimple, positive lens 24 and then through a pair of plane, parallelwindows 26, 28. Each of the plane, parallel windows is mounted to agalvanometer motor and positioned orthogonally to one another. The onlydifference between this third embodiment and the second embodiment isthat the second embodiment has a pair of galvanometer mirrors to deflecta laser beam through a scan lens (F-Theta type).

In the preferred, second, and third embodiments, the resulting focusedlight is directed onto a material such as a metal, plastic, glass orceramic for machining. In the preferred embodiment the meniscus lensesare mounted to galvanometers and oriented orthogonally to one another.The focal length of the lens combination of the two lens system isdefined by 1/f=1/f₁+1/f₂−t/f₁f₂, where f₁ is the focal length of thefirst lens, f₂ is the focal length of the second lens and t is theseparation between lens 1 and lens 2 where it is assumed for simplicityof description that the lenses used fall within the “thin lens” regime.The two lenses are tilted and naturally introduce specific lensaberrations such as coma, astigmatism and spherical aberration.Accordingly, the design of the lens curvatures, thickness and materialare optimized to minimize said lens aberrations at the designed radialdisplacement from the optical axis.

An inverted positive meniscus lens pair produces the least aberrationsfor the preferred embodiment. It is desired to have a high index opticalmaterial to facilitate longer radius of curvature surfaces and keep theoptical elements as thin as possible to further minimize theaberrations. It will be obvious to those skilled in the art that otherlens curvatures can be used besides inverted, positive meniscus lenses,e.g., a pair of plano-convex lenses; pair of double convex lenses, etc.In the preferred embodiment it has been determined that the inverted,positive meniscus lens pair provides minimal aberrations and bestoptical performance.

The tilting of each of the two meniscus lenses causes the laser beam tobe displaced in a controlled way from the optical axis. The amount ofdisplacement is dependent upon the power of the designed lenses and theangle that the lenses are tilted about the optical axis and orthogonallyto one another. In the preferred embodiment where the lens material issapphire with an index of refraction of 1.796, a combined lens pairfocal length of approximately 200 mm and a tilt angle of ten degrees(10°) of each lens, orthogonally, causes a radial shift of the focusedspot by >170 microns. Tilting the lenses beyond ten degrees causes thecoma to become too great for usefulness. As the lenses are tilted, theposition of the focal spot changes along the optical axis due tospherical aberration. This shift can be compensated in a well-knowncontrolled way, as is know to those skilled in the art by adding amotorized zoom lens system before the lens pair or by moving theworkpiece along the Z axis.

The preferred embodiment has a limited field that is determined by thefocal length of the lens pair and the angle of rotation, but nonethelessprovides very high precision capability of features below 500 microns insize in a very simple opto-mechanical configuration.

Another variation of the design places plane parallel plates onto thegalvanometers of the preferred embodiment, instead of the inverted,positive meniscus lenses, and positioned after a typical galvo-basedlaser scanner (a galvanometer mirror pair and an F-Theta lens). Theparallel plates permit the controlled shift of the laser beam passingthrough the scanner system and F-Theta lens. The plates are “thin” sothe introduced aberration is minor spherical aberration and allowsaccurate machining of finite features over the large area of thescanner/F-Theta system which can range from a few millimeters tohundreds of millimeters, depending upon the rotation angle of the galvoscan mirror and the focal length of the F-theta lens. Through softwarecontrol of the two galvo pairs, features can be accurately machined overa field range limited only by the scanner/F-Theta system used.

FIG. 6 depicts an array of nominally 155 μm diameter holes with acorresponding sigma of 0.049 machined with the second embodiment of thenovel device.

FIG. 7 depicts the same holes as depicted in FIG. 6, machined with aScanLab Hurryscan II scanner (F-theta lens=100 mm) of the prior art,having a sigma of 2.297.

A comparison of FIGS. 6 and 7 indicates that the novel deviceconsistently produces highly regular circular holes and that the priorart device does not.

FIGS. 8A-D respectively depict a circle, square, triangle and anirregular shape formed by a second triangle formed on top of a firsttriangle, machined in 80 um thick stainless steel by a second embodimentof the novel structure disclosed herein and having a nominal featuresize of 150 μm.

FIGS. 9A-D respectively depict a circle, square, triangle and anirregular shape formed by a second triangle on top of first triangle,machined in stainless steel with a calibrated ScanLab Hurryscan IIscanner (F-theta lens=100 mm) of the prior art. The nominal size ofthese features is about 150 μm.

A comparison of FIGS. 8A-D and FIGS. 9A-D indicates that the geometricalshapes formed in stainless steel by the second embodiment of the noveldevice are substantially true to idealized shapes and that thegeometrical shapes formed by a prior art device deviate substantiallyfrom the desired ideal shape.

It will be seen that the advantages set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween. Now that theinvention has been described,

1. A device for laser machining very precise geometric features over alarge processing window, comprising: a simple, positive lens; a pair ofplane, parallel windows; each of said plane, parallel windows beingmounted to a galvanometer motor and positioned orthogonally to oneanother; a laser beam having an optical axis; said laser beam adapted topass through said simple, positive lens and then through said pair ofplane, parallel windows; whereby a focused laser beam is displaced in acontrolled manner from said optical axis.